In the past, as a dilution sampling system that dilutes collected exhaust gas with diluent gas such as air to measure the concentration of a component contained in the diluted exhaust gas, a constant volume dilution sampling apparatus (CVS) has been widely used.
In this CVS, as disclosed in JP 06-341950, on an upstream side of a flow rate control part including a critical flow venturi and a suction pump, an exhaust gas mixer for uniformly stirring/mixing exhaust gas and diluent gas is provided.
Also, the conventional exhaust gas mixer is a mixer of a cyclone system, and provided with: a mixer main body that has inside a hollow part extending in a predetermined axial direction; a gas introduction pipe that is connected to a side wall of the mixer main body to introduce the exhaust gas and the diluent gas into the hollow part; and a mixed gas lead-out pipe that is inserted into the mixer main body and provided with a central axis coaxially with the central axis of the hollow part.
However, in the exhaust gas mixer having the above-described configuration, the central axis of the mixed gas lead-out pipe and the central axis of the hollow part are provided coaxially with each other, so that a space between the mixer main body and the mixed gas lead-out pipe has a constant cross-sectional area in a spiral direction, and therefore a spiral velocity of the gases introduced from the exhaust gas introduction pipe is excessively increased to cause noise or vibration. Note that it is generally said that noise is proportional to the sixth to eighth power of a flow velocity. Also, there is a problem that the increase in gas spiral velocity causes resonance in the exhaust gas mixer to make the noise or vibration larger.
In addition, it is possible to decrease the flow velocity of the mixed gas flowing into the exhaust gas mixer; however, in the exhaust gas mixer used for the CVS, a flow rate is controlled so as to meet a constant flow rate through the critical flow venturi, and therefore the flow velocity of the mixed gas from the gas introduction part cannot be decreased.